The invention is based on a transformer comprising a primary winding and at least one secondary winding, which are arranged in the form of conductor tracks on one or more layers of a flat carrier, in particular films or printed circuit boards. Transformers of this type are used for example in switched-mode power supplies having very high switching frequencies, in particular frequencies of more than 50 kHz, thereby enabling the transformer to be kept very small.
When the transformer is reduced in size, it is necessary to ensure sufficient insulation between the primary side and the secondary side if the transformer is not intended to be potted. This is particularly important for switched-mode power supplies having mains power supply isolation, for which safety regulations demand a creepage path for electric surface currents from the secondary side to the primary side of at least 6 mm, which must be adhered to throughout the entire transformer. With given dimensions of the transformer core, the available useful volume for the windings is thereby restricted. The safety regulations consequently lead to larger components and higher power losses due to increased non-reactive resistance and due to reduced magnetic coupling.
Known transformers of this type, illustrated in FIGS. 1a, 1b, contain for example a core with two core parts K1, K2, primary and secondary windings W1, W2 being arranged in the openings therein. The windings are arranged as conductor tracks L on non-conductive layers F of a flat carrier, a plurality of layers in each case lying one above the other, depending on the number of turns required. A plurality of layers P are in this case serially interconnected by plated-through holes T2 and have two external terminals T1, T3. In order to achieve sufficiently long creepage paths between the various windings, the outer and inner edges R of the layers P must remain free, in other words are not permitted to have any conductor tracks. The conductor tracks L are produced as copper tracks on the layers P using etching technology, for example. Figures 1a, 1b are sectional drawings corresponding to a central horizontal and a central vertical plane of the transformer.
The object of the present invention is to specify a compact transformer of the type mentioned in the introduction which has improved electrical properties.